The invention relates to a method and arrangement for cooling a hot surface, in particular the jacket of a metallurgical vessel, wherein a liquid cooling medium is atomized by means of a plurality of nozzles in a hollow space, which surrounds the surface and is opened towards the atmosphere.
Pyrometallurgical processes, as a rule, take place in vessels comprising a jacket of steel plate which is lined with refractory material in order to put up with the high process temperatures prevailing in the interior of the vessel. However, this lining does not always offer the opportunity of providing the low temperatures required for the strength of the steel jacket. In order to avoid wall temperatures that are too high it is known to cool the vessel wall by forced cooling with the aid of gaseous and/or liquid coolants, for instance, by means of surface irrigation cooling.
According to U.S. Pat. No. 4,815,096, whose disclosure is incorporated by reference thereto, water is sprayed in large amounts on the hot surface of the jacket of a metallurgical vessel within a chamber which closedly surrounds the hot surface and is subjected to an overpressure. The cooling water collecting in the chamber, which has not evaporated or condensed, is guided in circulation. However, difficulties have to be overcome, due to the collecting of the cooling water, when tilting the metallurgical vessel with the hot surface, primarily in order to avoid a loss of pressure within the chamber.
It is true that this cooling, and also surface irrigation cooling, offer advantages in terms of excellent heat transmission conditions; however, such cooling also involves the considerable disadvantage of the cooled vessels having to be as stationary as possible because of the required waste water collection means. The application of this type of cooling with tiltable converters or tiltable lids, etc., is feasible only to a limited extent. Besides, the good cooling obtained by a cooling of this type is not desired, anyway, since, as a result, quantities of heat that have to be produced in the process in a cumbersome and expensive way must be conveyed off.
Since no hot cooling medium would then have to be conveyed off any longer, it would be possible to eliminate these disadvantages by cooling by means of a gaseous medium. Yet, the main disadvantage involved therein is the very low heat capacity of the gaseous media, i.e., very large amounts of gas are required and, moreover, the heat transmission coefficients are low, thus requiring high flow speeds.
To avoid these disadvantages, it is known from EP-A-0 044 512 to spray water on the hot surface. The amount of water to be sprayed is a function of the water evaporated on the hot surface so that no backflowing cooling water need be collected. The coolant is sprayed in a closed chamber and the condensed water is collected and recycled.
In doing so, the cooling water must be supplied at a high speed and in large amounts in order to break the boundary layer on the hot surface. Although EP-A -0 044 512 already speaks of a droplet size of 100 .mu.m at the most and of controlling the amount of sprayed-on water by means of a microprocessor on the basis of measured temperature values, too strong local and temporal cooling cannot be avoided. Consequently, it is necessary to provide means for turning on/turning off the spray which is controlled via thermocouples. However, the temperature changes occurring in the course of time, i.e., the high time-dependent temperature gradients, are dangerous with a view to excessive temperature stresses and symptoms of fatigue of the vessel jacket. Furthermore, cold spots are created within the spraying cone of the nozzles oriented directly towards the hot surface, and result in great temperature differences and hence high stresses.
EP-A 0 393 970, suggests a variant of the cooling method described above, wherein spraying is effected not directly on the surface to be cooled, but somewhat parallel to the same. According to that document, good uniform cooling effects are said to be obtained while avoiding a too abrupt cooling and by using only a slight or small number of nozzles.
However, there is a disadvantage to be seen in the mode of spraying of the cooling medium. According to EP-A -0 393 970, the coolant is sprayed by means of a binary nozzle, i.e., by aid of a gaseous medium. The following holds for the exit speeds of binary nozzles: The carrier gas emerges from the spraying nozzle, following the thermodynamic laws. Theoretically, a carrier gas reaches Laval speed, i.e., a speed near ultrasonic speed. With normal physical conditions prevailing, this means a speed of about 300 m/s. The water is injected into this stream under pressure and is entrained, hardly reducing the speed. As a result, such a nozzle has a very wide streaming range in which this speed is high and the streaming range itself extends up to 4 m. When impinging on a surface to be cooled which is approximately normal to the direction of the stream, a very good cooling effect is reached in a limited area. Since this results in cold spots, which must be avoided for reasons of strength as pointed out above, the nozzles according to EP-A -0 393 970 are arranged in a manner that the stream is ejected approximately parallel to the surface to be cooled. However, since the stream spreads conically and still has a very high speed at the point at which it impinges on the wall to be cooled, the risk of forming cold spots continues to exist.
Again, one is forced to provide turning on and off of the coolant supply in response to signals from thermocouples. This results in a strong dependence of the temperature of the surface to be cooled on time, i.e., the temperature fluctuations, which occur, exhibit a very large gradient as a function of the time.
Concerning the efficiency of binary nozzles injecting parallel to the wall to be cooled, as described in the prior art, in respect of their heat transformation, it is to be noted that the major portion of the cooling effect of the cooling medium is lost. This is because, as already pointed out, an external boundary wall, which is relatively cold, gets strongly involved in the cooling process due to the conical spreading of the emerging stream, which cannot even be prevented by specially designed flat nozzles. A considerable quantity of the gas/water mixture precipitates on this cool external boundary wall. This water only slightly participates in the heat transformation and runs off along the external wall. This may also cause condensation of already formed vapor.
In case such cooling is applied to a steelworks converter, two conduits (coolant and gaseous medium) are to be provided through a rotary introduction that is provided on the carrying trunnion of a converter carrying device (carrying ring). This involves increased expenditures both in terms of construction and in terms of maintenance.